1. Field of the Invention
The invention relates to processes for drying tobacco and, more particularly, relates to a process for reducing the moisture content of expanded tobacco.
2. Brief Description of the Prior Art
In the manufacture of cigarettes and like articles, it is the usual practice to reduce tobacco, the term being used herein to include both lamina and stems, to a particle size appropriate for preparing cigarettes. The moisture content of the tobacco is generally increased prior to this size reduction to minimize shattering and provide a material of more uniform particle size. In order to permit subsequent processing, e.g., formation of the cigarette rod, it is necessary to reduce the moisture content of the tobacco to a level below that at which size reduction is conducted.
In general, the prior art discloses drying of cut tobacco from an initial moisture content, usually about 16 to 35 percent in the case of lamina and about 20 to about 60 percent in the case of stem, to a moisture range of about 12-15 percent by passing the tobacco through hot air under time and temperature conditions adequate to effect the desired moisture reduction. The prior art discloses various apparatus and procedures for effecting this drying. For example, U.S. Pat. No. 3,357,436 discloses drying cut tobacco, having an initial moisture content of 16-35 percent, to a final moisture content of approximately 13 percent by exposing the tobacco to air heated to a temperature of 150.degree.-600.degree. F., the air having a water content of at least 10 percent by weight.
In recent years it has become a widespread practice in the tobacco industry to expand or "puff" cut tobacco prior to its incorporation into cigarettes. Expansion processes produce tobacco having a reduced density or increased filling power, i.e., an increase in the volume occupied by a given weight of tobacco, permitting improved quality and economics and reduced "tar" and nicotine deliveries. Numerous techniques are described in the prior art for effecting tobacco expansion. In general, tobacco expansion is achieved by impregnating tobacco with water, an organic liquid, carbon dioxide, ammonia, or some combination thereof, followed by subjecting the impregnated tobacco to increased temperature and/or reduced pressure conditions. In prior art techniques for then drying the expanded tobacco, much of the advantages attributable to the expansion is lost or reduced due to shrinkage which occurs during the drying process. A process by which expanded tobacco could be dried to a desired level, while minimizing any concomitant loss in filling power due to shrinkage, is of substantial benefit.
By the method of the present invention, one may reduce the moisture content of expanded tobacco to a desired level, while minimizing loss in filling power.